1. Field of the Invention
The present invention relates to heat-curable organopolysiloxane compositions which exhibit improved adhesion to unprimed substrates.
2. Description of the Prior Art
The problem of adhering heat-curable or cured organopolysiloxane elastomeric compositions to various substrates is nearly as old as the silicone art itself. One solution to this problem has been to treat the surface of the substrate with a reagent such as a strong acid to render the treated surface more receptive in the adhering process. Another solution to this problem has been to prime a surface of the substrate with an adhesion-promoting primer composition and to subsequently apply the heat-curable or cured organopolysiloxane composition to the resulting primed surface. Considerable effort has been devoted to this primer method and significant progress has been made. However, many of the primer compositions that are used in the organosilicon elastomer art are difficult or inconvenient to prepare. The need to apply a primer composition to a substrate in a separate step is also, in many cases, inconvenient and time-consuming.
It is known to add certain adhesion-promoting materials to peroxide-curable organopolysiloxane compositions to obtain better adhesion of the organopolysiloxane composition to an unprimed substrate. U.S. Pat. No. 3,730,932 to DeZuba, et al. teaches a self-bonding, heat-curable silicone rubber composition comprising a diorganopolysiloxane polymer, a curing catalyst and certain maleates and/or silymaleates. U.S. Pat. No. 3,794,612 to Edwin P. Plueddemann teaches the addition of diallylphthalate, alone or in combination with vinyl- or allyl-substituted trialkoxysilanes, to curable silicone rubber compositions to improve the adhesion of the elastomer to substrates such as wood, metal and plastics. U.S. Pat. No. 3,813,364 to DeZuba, et al. teaches a self-bonding, heat-vulcanizable silicone rubber composition comprising an organopolysiloxane polymer, a curing catalyst and an additive selected from cyanurates, isocyanurates, silyl-cyanurates and silyl-isocyanurates.
It is also known to add certain olefinicallyunsaturated organosilicon materials to heat-curable oganopolysiloxane compositions to improve the curing process thereof. Thus, U.S. Pat. No. 3,341,489 to Simpson claims a composition curable from 80.degree. C. to 650.degree. C. comprising an organopolysiloxane polymer having a viscosity of at least 100,000 centipoise at 25.degree. C., a filler, a peroxide curing catalyst and an olefinically unsaturated organosilane of the formula ##STR1## wherein R' is an olefinically unsaturated monovalent hydrocarbon radical, R" is hydrogen or a monovalent hydrocarbon radical free of olefinic unsaturation, X is alkoxy, alkoxyalkoxy, alkoxyaryloxy or acyloxy and c is an integer from 1 to 3.
Room-temperature-curing organopolysiloxane elastomers have found utility in the marketplace because they are different from and complementary to the heat-curing silicone rubber compositions. It is unexpected to form a mixture comprising an organic-peroxide-curable silicone rubber composition and a room-temperature-curable silicone elastomer to obtain a heat-curable silicone rubber composition having improved unprimed adhesion.